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Full text of "Treatise On Applied Analytical Chemistry(Vol-1)"

FATTY SUBSTANCES  (GENERAL METHODS)            381

time of contact of the fatty substance with the iodine solution is reduced
to about an hour for non-drying or but slightly drying oils and fats and to
about two hours for the others. The calculation is made as with Hubl's
method.

EXAMPLE: For 0-352 gram of olive oil 24-90 c.c. of thiosulphate were
required, and in the check experiment, 48-80 c.c. The quantity necessary to
decolorise the excess of iodine is, therefore, 48-8 — 24-9 = 23-90 c.c.

Assuming that 0-2 gram of iodine corresponds with 16-5 c.c. of thiosulphate,
i.e., 0-01212 gram with i c.c., the amount of iodine absorbed by 0-352 gram
of the oil is 23-9 x 0-01212 =0-2896 gram, so that the iodine number is

X ioo

= 82-27.

0-325

As a rule the iodine numbers obtained by the second method (Wijs's) are
rather higher than those given by Hubl's method.

The determination of the iodine number is of great importance for the analysis
of fatty substances, since it serves to characterise many of them and to indicate
if they are pure or mixed. Drying oils (linseed, hempseed, walnut, poppy-
seed, madia, Japan wood, etc.) and fish oils (sardine, anchovy, herring, cod)
have very high iodine numbers, which usually exceed 120. The non-drying
oils (olive, arachis, almond) have iodine numbers below ico. The semi-drying
oils (colza, cottonseed, sesame, maize) have intermediate values. Iodine num-
bers between 30 and 60 are usually shown by vegetable fats, excepting coconut
oil, palm-kernel oil and certain so-called vegetable waxes (myrtle, Japan),
which have values below n. With the animal fats the iodine number is not
very high, being usually less than 90.

With each individual fat the iodine number may vary between fairly wide
limits, in accordance with the method of preparing the fat, with the degree
of maturity of the fruit or seed yielding it, with the conditions of storing and
age of the fat, etc. Very wide variations are, however, exceptional, and in
most cases the iodine number keeps moderately constant (see, for example,
Olive Oil), so that it may be used for the approximate determination of the
respective quantities of fats in a mixture of two of known character—the calcu-
lation being made according to the law of mixtures.

The causes of pronounced variations in the iodine number are various, but
of especial importance are the age and storage conditions of the fat. In general,
old and badly stored (rancid) fats have iodine numbers lower than those of
the corresponding fresh and well-kept fats ; this is notably the case with drying
oils, which readily absorb atmospheric oxygen.

13. Absolute or " Inner " Iodine Number

This represents the weight of iodine absorbable by ioo parts of the
liquid fatty acids obtainable from a fatty substance.1 It is determined
on the liquid fatty acids isolated by Tortelli and Ruggeri's process (see later :

18).

Ten or fifteen drops of the liquid acids, just prepared, are weighed in
a bulb of thin glass and the iodine number then determined as indicated
in the preceding article (12).

1 It is the liquid portion of the fatty acids, separated as described later, that con-
tains the unsaturated fatty acids (oleic, linoleic, linolenic, etc.), which have the property
of fixing iodine, so that the iodine number of this portion is properly called the absolute
iodine number.ults taken, provided t hat these do not differ greatly.quid remains clear.r.